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Old 02-27-2010, 07:42 PM  
D725A
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Default Question for Nestor on priming plaster

hi Nestor, I have read a few great threads of yours which have been very helpful. My situation is that we have removed wallpaper and the glue from the plaster wall in the bedroom of our 1924 house. There appear to be at least three layers of paint--probably more, and i'm guessing some were oil. The walls were lightly sanded after i took out wallpaper and glue.

I had a plasterer come by and repair the major cracks for which he used plaster weld; he then used a mix of standard joint compound and easy-sand setting joint compound, mesh, etc. there were many small scraped areas of the wall, where underlayers of paint were visible, most of which also had plaster weld applied before spackling. (The glue was so stuck on the wall in sheer desperation i sometimes had to scrape it off.) I gather I should spot-prime these areas before priming the entire wall.

The dilemma is whether to use an oil-based primer to stick better on these old walls or some other strong latest primer. all the hardware guys say oil. I don't mind that, but the people at buildingscience.com maintain that in my NY summer hot/humid climate, oil paint is a vapor barrier, and during summer the oil paint would act as a vapor barrier which would prevent moisture from drying to the inside. So i'd prefer a latex primer if possible.

I'm also curious to know if it's a big deal if the plasterer did not put plaster weld on all the little scraped areas before spackling. These are inches small areas.

Thanks.



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Old 02-28-2010, 12:27 PM  
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D725A:

I don't know what "Plaster Weld" is, but I expect it's just Plaster of Paris mixed with glue or something to make it stick better.

The humidity issue isn't a factor to be considered in my view. Airborne humidity isn't really going to migrate through a 3/4 inch thick plaster wall to any great extent anyhow, especially if there's still old oil based paint on those walls. So what you put over that wall really wouldn't matter in my view. There's gonna be way more airborne humidity migrating into/out of your walls through every light switch, light fixture and electric outlet than there's gonna be through the plaster itself. Ditto for the cracks under baseboards and stuff.

Since the paint on these walls was probably put on a long time ago (prior to the mid-1980's) there's a real good chance that the paint on the walls is a linseed oil based paint, and you could have also just cleaned the walls with TSP to ensure good adhesion of the top coat rather than sand them. Even if they're already sanded,
you can still clean with TSP to improve adhesion if you want. Just be sure to rinse the TSP off completely with clean water and allow plenty of time to dry (especially on exterior walls) before priming.

I prime with Zinssers Bullseye 123 latex primer in my building simply because it sticks well to just about everything, including oil based paints, but if the walls have been sanded (or cleaned with TSP) you can use any latex or oil based primer you want. 123 has a smell to it that some people don't like, and it can be hard to get off your skin or clothes. It's true that oil based paints adhere to smooth surfaces better, but if you've sanded the old paint, then both oil or latex primers will stick well. (It's actually the increase in surface area caused by sanding that results in the improved adhesion. There's the same adhesion between the old paint and the top coat, but because the area over which that adhesion occurs is larger on a sanded surface, the top coat holds on better if the substrate is sanded (and presents a larger surface area).

No, you can fix cracks smaller than an inch wide with some base coat plaster. If you're starting to do this kind of work, you might want to buy some base coat plaster. For you, one bag would be a lifetime's supply, so maybe you can just phone up some plaster or drywall contractors and ask what kind of base coat plaster they use and if they'll sell you some Perlite Admix Hardwall. I like it better than Structo-lite. Maybe Google Perlite Admix Hardwall and find out who makes it, and try to get some. It's cheap. Or, phone around to the drywall wholesalers to see what base coat plasters they sell. If they have a torn bag, they'll probably be willing to sell what's left in it super cheap.

If you've got plaster walls, then you pretty well need to get into the practice of repairing them with both base coat plaster and joint compound; although you COULD just use joint compound for the whole repair, too.

Yes, I've found that when I repair nail holes, the remaining joint compound around the repair will draw in more primer and there will be a dull areas on the wall where more primer was absorbed by the repair plaster. So, yeah, maybe go around with a 3 inch roller and pre-prime all of the bare plaster areas. If the painted areas have been well sanded, or cleaned with TSP, you don't really need to prime those.

Ideally, if you could open an account at w w w.photobucket.com and beg, borrow or steal a digital camera so you could take pictures of the problem areas and post them, the advice I give you would be better cuz I'd know better how to repair each problem by seeing it.

Not sure if I answered all your questions or not. Ask again if I didn't.



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Old 02-28-2010, 02:10 PM  
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Thanks for taking the time to answer. FYI Plaster Weld, made by Larsen, is a pink bonding agent used on an old wall before repair plastering begins.

I too have found Zinnser's bull'seye 1-2-3 to work well.

Watching the plasterer work i have come away with new appreciation for the skill involved with the blades and trowels. The major repair here aside from the walls was the ceiling, which had cracks and worse, paper tape hanging down from where the previous owner had had some cracks repaired. After i scraped and sanded the ceiling, my guy plastered in three layers of decreasingly heavy plaster--from joint compound to a mix of that with setting joint compound Easy Sand. Looks good so far.

You have answered most of my questions, thank you. Only thing further I have is how long to wait until applying the primer--especially to the ceiling, and then how long after that for the finished coat. I have read on other sites that plaster and paint can take a really long time--months--to fully dry. note that the ceiling is underside the attic floor, so it probably gets quite cold at times in the winter.

Thanks again.

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Old 02-28-2010, 02:34 PM  
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D725A:

Plaster and joint compound will both become lighter as they moisture evaporates from them. As long as there is any moisture inside the plaster or joint compound, then the surface of the repair will look darker. (Just like wet sand looks darker than dry sand.)

Once the surface of the plaster or joint compound is white, then you can prime.

A pre-mixed joint compound can take months to dry if it's in a cold place. However, most joint compounds that come as powders in a bag have a chemical set that kicks in after anywhere from 20 minutes to 2 hours after mixing with water. Once that chemical set kicks in, the joint compound becomes quite stiff quite quickly. It will still "look" wet, tho, if it hasn't dried by then. In both cases, the long period of staying wet won't harm either a premix or powder joint compound.

Also, the thinner the coat of plaster or joint compound, the faster it dries. So, if you feel that drying is gonna be a problem, put the joint compound on in thin coats and use a hair dryer to dry it.

Oil based paints can stay wet for months if they're cold too. However, when they warm up, they will dry normally. The long period of not drying won't affect the paint film once it does start to dry.

Latex paint can't do that. The film forming mechanism in latex paints is a process called "coalescence" whereby the hard plexiglas resins are softened by a water soluble solvent (called a "coalescing solvent") dissolved in the paint. The whole process requires the water to evaporate from the paint before any significant amount of the coalescing solvent does. If the latex paint doesn't dry because of high humidity, that solvent would still continue to evaporate from the paint film, and the result would be ruined paint. Similarily, if the latex paint didn't dry because of cold temperatures, the plastic resins in the paint would be too hard to be sufficiently softened by that solvent to deform and create a continuous film, and again the result would be ruined paint. In both cases, the paint, when it did dry, would most likely dry to a coloured powder that you could rub off easily with your finger.

Anyhow, if you have a digital camera, try to open an account (it's free) at any web site that allows you to post pictures. You always get better advice when the guy on the other end can see the same problem you can.

G'Luck

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Old 02-28-2010, 02:40 PM  
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I'm surprised in a seven hundred word essay there was no mention of "lead paint" or the hazards of sanding such.

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Old 02-28-2010, 02:42 PM  
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thanks nestor, I think this site allows direct attaching of digital images, so that shouldn't be a problem. I just wonder if anyone would be able to tell anything from a photo of a white wall or ceiling. but if anybody could, I'm sure it would be you. Thanks much for your time and expertise.

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Old 02-28-2010, 03:45 PM  
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I'm surprised in a seven hundred word essay there was no mention of "lead paint" or the hazards of sanding such.
Yeah. I'm bad.
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Old 02-28-2010, 03:50 PM  
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That's OK, I won't hold it against you. You will however be receiving 12 Mr. Wizard demerits for your oversight.

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Old 02-28-2010, 06:50 PM  
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That's OK, I won't hold it against you. You will however be receiving 12 Mr. Wizard demerits for your oversight.
and when you get to 18 Mr. Wizzard demerits you get a bucket of chicken, 1 pint of cole slaw, biscuts and a bowl of grits.
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Old 02-28-2010, 07:39 PM  
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Originally Posted by Bud Cline View Post
there was no mention of "lead paint" or the hazards of sanding such.
I thought it was amnesia from fumes of undetermined origin. You own any animals?

"A Swedish animal protection inspector attempting to rescue 20 abandoned cats was hospitalized with amnesia after she was poisoned by fumes from rotting cat urine and excrement, the Gothenburg Environmental Agency said."


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